1. Field of the Invention
The present invention is related to a method for fabricating Group III nitride-based compound semiconductor products on Group III nitride-based compound semiconductor substrates and, in particular, to such a method including surface treatment of Group III nitride-based compound semiconductor substrates prior to epitaxial growth. The semiconductor substrates are for production of Group III nitride-based compound semiconductor devices, thick films or ingots.
As used herein, the term “Group III nitride-based compound semiconductor” encompasses a semiconductor represented by the formula AlxGayIn1−x−yN (0≦x≦1, 0≦y≦1, 0≦x+y≦1); such a semiconductor containing a predetermined element so as to attain an n-type/p-type conduction; and such a semiconductor in which a portion of a Group III element is substituted by B or Tl, and a portion of the Group V element is substituted by P, As, Sb, or Bi.
2. Background Art
In recent years, Group III nitride-based compound semiconductor substrates have been available for use as a substrate on which a Group III nitride-based compound semiconductor light-emitting device is formed. For example, a GaN ingot is produced through a variety of techniques, and the ingot is cut to pieces having a thickness of interest, thereby provided as commercial products. When such an ingot is cut, a cut face serves as a main face of a substrate and, therefore, undergoes mechanical polishing and/or chemical mechanical polishing.
Currently, no simple technique has been realized for planarizing, with high precision, a surface of a Group III nitride-based compound semiconductor substrate such as a GaN substrate, since Group III nitride-based compound semiconductors per se have very strong resistance to generally performed wet etching. Thus, when a strong etchant such as an aqueous alkaline solution is employed, the etched surface is provided with undesirably deep pits.
In order to solve the problem about planarizing Group III nitride-based compound semiconductor substrates, attempts have been made to perform heat treatment, or a combination of heat treatment and other treatments. Japanese Patent Application Laid-Open (kokai) Nos. 2003-327497, 2005-136311 and 2004-273484 disclose such techniques.
The techniques disclosed in the first and the second patent documents each include heating a surface of a GaN substrate in an ammonia-hydrogen atmosphere at 1,000° C. or higher for 10 minutes or longer. The third patent document discloses the technique to set the heat treatment temperature lower than the following GaN vapor growth temperature and to reduce the supply of material gas containing a group III element lower than that at the following epitaxial growth.
When the present inventors performed those heat treatments disclosed in the first and the second patent documents, solidified products of Ga droplets (hereinafter referred to as “solidified Ga droplets”) were observed on the surface of the GaN substrate by an atomic force microscope (AFM) or a metallographical microscope. The formation of solidified Ga droplets indicates that nitrogen evaporated from the surface of the GaN substrate during heating, and remaining Ga atoms aggregated. When a GaN substrate having solidified Ga droplets on the surface thereof is employed as an epitaxial growth substrate, the film grown on the substrate fails to have uniformly high crystallinity over the entire wafer surface. In other words, when a GaN substrate is treated by a technique disclosed in the patent documents, the formed epitaxial film fails to have high crystallinity, and a semiconductor device layer formed on the epitaxial film fails to exhibit excellent characteristics. In addition, a method disclosed in the third patent document causes a problem such as enhancing decomposition of GaN substrate and appearance of Ga particles since epitaxial growth temperature is higher than that of cleaning process.